Coiled tubing overbalance stimulation system

ABSTRACT

Formation zones of interest may be isolated and stimulated by substantially instantaneously providing a pressure equal to or greater than the fracture pressure of the targeted zone. A treatment fluid is accumulated in coiled tubing until the pressure is equal to or greater than the fracture pressure of the targeted zone. The accumulated pressurized treatment fluid is released substantially instantaneously at the targeted formation zone. The accumulated pressurized treatment fluid may be substantially simultaneously released with the detonation of a perforation gun.

FIELD OF THE INVENTION

The present invention relates in general to stimulating a well to increase the rate of flow into or out of the surrounding formation and more particularly to a method and apparatus for extreme overbalance stimulation of the formation surrounding a well utilizing coiled tubing.

BACKGROUND

Well stimulation refers to techniques used for increasing the productivity of the formations surrounding the well. For production wells, it is important to increase the rate at which fluid flows from the formation into the wellbore. For injection wells, it is often important to increase the rate of injection of fluid into the surrounding formation.

The region of the formation proximate the wellbore is very often the most significant restriction to the well's productivity. This region is frequently damaged during the drilling of the wellbore creating resistance to fluid flow through the region. The resistance to fluid flow near the wellbore is defined by “skin factor.” Skin factors are measured by measuring bottom hole pressures in a well under differing flow conditions. A positive skin factor indicates that the region near the wellbore is more resistive to flow than the formation farther away from the wellbore. A negative skin factor indicates that the region near the wellbore is less resistive to fluid flow than the formation farther away from the wellbore. The resistance to fluid flow can be reduced by stimulation.

The two primary methods for treating these damaged regions or zones to stimulate productivity, include chemical stimulation and fracturing. In chemical stimulation, fluids such as acids or other chemicals are injected into the formation to interact with the rock matrix to increase the permeability of the formation. In hydraulic fracturing, a fluid under a pressure greater than the stress of the formation is injected forming fractures in the rock matrix. Typically, proppant is included in the fracturing fluid so as to be lodged in the fractures to maintain the created permeability to fluid when the fracturing pressure ceases. It is desired at times to combine chemical stimulation with fracturing, for example with acid fracturing. In certain circumstances fractures may be created extending hundreds of feet from the wellbore.

It is more economical, and at times only necessary, to create fractures near the wellbore. One method of creating these near-wellbore fractures is by overbalance stimulation, wherein pressure greater than the formation's fracture pressure is suddenly released proximate the formation. Prior art methods have been utilized, however, these methods are expensive and often uneconomical and thus not feasible for multi-formation wells. The prior art systems fail to provide a mechanism for selectively treating formation zones in uncompleted, open hole, wellbores or slotted liner completions.

Therefore, it is a desire to provide an extreme overbalance stimulation method that addresses the shortcomings of the prior art devices. It is a still further desire to provide a overbalance stimulation method that utilizes coiled tubing as the main treatment string.

SUMMARY OF THE INVENTION

In view of the foregoing and other considerations, the present invention relates to an extreme overbalance stimulation method utilizing coiled tubing as the main treatment string.

Accordingly, an extreme overbalance stimulation system and method is provided. An embodiment of the overbalance stimulation method includes positioning an assembly connected to coiled tubing adjacent a target formation zone and sealing between the coiled tubing and the formation to isolate the target formation zone from the rest of the wellbore. A desired pressure is accumulated in the coiled tubing with a treatment fluid. The desired pressure is at least equal to the fracture pressure of the target formation zone. The accumulated pressure is substantially instantaneously released through the assembly at the target formation zone.

In another embodiment, the extreme overbalance stimulation method includes positioning an assembly connected to coiled tubing adjacent a target formation zone, wherein the assembly includes a perforating gun. A desired pressure is accumulated in the coiled tubing with a treatment fluid. The desired pressure is at least equal to the fracture pressure of the target formation zone. Upon detonating the perforating gun the accumulated pressure is substantially instantaneously released through the assembly at the target formation zone. Creating a temporary path of least resistance, ensuring preferential flow toward the targeted zone in open hole and slotted liner completions.

The system may include a coiled tubing unit having coiled tubing adapted for disposing in a wellbore and a pump (or several pumps) for providing a pressurized treatment fluid in the coiled tubing. A pressure release device is in fluid connection with the coiled tubing, the pressure release device being connected to a port positionable proximate a targeted formation zone. The pressure release device is operatable between a closed position for maintaining the pressurized treatment fluid within the coiled tubing, and an open position for substantially instantaneously discharging the pressurized treatment fluid through the port. The system may further include a top sealing element adapted for sealing between the coiled tubing and the formation, and a bottom sealing element adapted for sealing between the coiled tubing and the formation, wherein the bottom sealing element is spaced from the top sealing element sufficient to isolate the targeted formation zone from the rest of the wellbore.

The foregoing has outlined the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description of a specific embodiment of the invention, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is an illustration of an embodiment of the overbalance stimulation system of the present invention set for stimulating a first formation zone;

FIG. 2 is a schematic illustrating the stimulation of the first formation zone of FIG. 1;

FIG. 3 is an illustration of the overbalance stimulation system set for stimulating a subsequent formation zone;

FIG. 4 is a schematic illustrating the stimulation of the second formation zone;

FIG. 5 is an illustration of another embodiment of the overbalance stimulation system of the present invention; and

FIG. 6 is an illustration of the step of stimulating a targeted formation system of the embodiment of FIG. 5.

DETAILED DESCRIPTION

Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.

As used herein, the terms “up” and “down”; “upper” and “lower”; and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements of the embodiments of the invention. Commonly, these terms relate to a reference point as the surface from which drilling operations are initiated as being the top point and the total depth of the well being the lowest point.

FIG. 1 is an illustration of an embodiment of an overbalance stimulation procedure and system using coiled tubing, generally denoted by the numeral 10. Overbalance stimulation system 10 includes a coiled tubing unit 14 having coiled tubing 16 for spooling into and out of a wellbore 18 via an injector head 20 (or other coiled tubing deployment apparatus) through a wellhead 22. A pump 24 is in fluid communication with coiled tubing 16 for providing a pressurized treatment fluid described in more detail below. As well-known in the art, pump 24 is further connected to a treatment fluid reservoir (not shown) as well as a high pressure treating line (not shown). The treatment fluid may include, singularly or in combination, a variety of fluids, including, but not limited to, gasses, liquids, acids, gels, foams, and slurries. It should be further recognized that the treatment fluid may carry a proppant.

Wellbore 18 may be substantially vertical or at any angle including horizontal. The earth surrounding the wellbore includes a formation having one or more formation zones that are targeted to produce fluid from or inject fluid into. In the present embodiment three target zones 26 a, 26 b, and 26 c are illustrated. Zones 26 may include perforations 28, or perforations may be created during the process of the present invention. Wellbore 18 may include casing 30, include a slotted liner, be an open hole completion, or be a combination of the aforementioned.

System 10 further includes a bottom hole assembly (BHA) or treatment assembly 32 in functional and fluid connection with coiled tubing 16. Assembly 32 includes a pressure release device 34, a discharge port 36 and an upper and lower sealing element 38, 40.

Sealing elements 38 and 40 may be packers, cups or any other elements or mechanisms adapted for isolating a formation zone 26 to be stimulated from the rest of wellbore 18. Pressure relief device 34 is adapted for maintaining treatment fluid 42 within coiled tubing 16 under pressure until it is desired to discharge fluid 42 through port 36. Pressure relief device 34 may be, but is not limited to, valves or frangible discs. Port 36 is positioned between sealing elements 38, 40.

FIG. 1 illustrates the system set in wellbore 18 for stimulating first formation zone 26 a. Although first formation zone 26 a is illustrated as the lower formation zone, the first formation zone may be any zone desired to be treated. Coiled tubing 16 is spooled through injector head 20 and wellhead 22 into wellbore 18 positioning BHA 32 proximate first zone 26 a. Sealing elements 38 and 40 are set between coiled tubing 16 and formation 26, substantially sealing zone 26 a from communication with the remaining portion of wellbore 18. Port 36 is positioned proximate first formation zone 26 a and pressure release device 34 is in the closed position, preventing fluid flow between coiled tubing 16 and wellbore 18 through port 36.

With port 36 of pressure release device 34 in the closed position, pressure is accumulated in coiled tubing 16. The desired pressure accumulated is at least equal to and preferably greater than the fracturing pressure for the targeted zone 26, shown as zone 26 a in FIG. 1. The fracture pressure of the zone varies with its depth and its geological characteristics. Often, an initial breakdown pressure greater than the fracture pressure is required to initiate the fractures. This higher initial breakdown pressure is due in part to the necessity of overcoming the tensile strength of the rock. Thus, the desired pressure accumulated may be equal to or greater than the initial breakdown of the formation of interest.

The pressure is accumulated by pumping treatment fluid 42 into coiled tubing 16 via pump 24. As described, treatment fluid 42 may be any fluid necessary and adapted for the fracturing the formation zone of interest. For example, it may be desired to utilize nitrogen in a coal seam well. Additionally, it may be desired to include proppant in treatment fluid 42 to maintain the fractures when the pressure is released, or follow the initial extreme overbalance stimulation with a fluid carrying proppant or other treatment substances.

FIG. 2 illustrates the step of fracturing zone 26 a utilizing extreme overbalance stimulation. Once the desired stimulation pressure is accumulated in coiled tubing 16, pressure release device 34 is activated to the open position in a manner to substantially instantaneously release the accumulated pressure through port 36 to act on formation 26 a. Pressure release device 34 may be activated by any suitable means. For example, the release device 34 may be a frangible disc activated by a dropped bar or ball. Similarly, the release device 34 may be a valve activated electrically, hydraulically or optically. Any means capable of activating the pressure release device 34 should be considered to be within the scope of the present invention.

The sudden, substantially instantaneous, release of the accumulated pressure equal to or greater than the fracturing pressure causes fractures 44 to be formed in zone 26 a. Seals 38, 40 isolates treatment fluid 42 and the accumulated pressure at zone 26 a. Further pumping of treatment fluid 42 or a subsequent treating fluid may be pumped after the shock treatment. The overbalance treatment may be repeated numerous times if a series of shocks are needed or desired.

With reference to FIGS. 2 and 3, once stimulation of zone 26 a is completed seals 38, 40 are released. Coiled tubing 16 is then spooled positioning port 36 of pressure release device 34 adjacent to the next formation zone targeted for stimulation, shown as 26 b herein. Seals 38, 40 are set substantially isolating zone 26 b from the rest of wellbore 18 and formation 26. With pressure release device 34 in the closed position, the pressure, via treatment fluid 42 and pump 24, is accumulated in coiled tubing 16 at a pressure equal to or greater than the fracture pressure of formation zone 26 b.

Once the desired pressure is accumulated, pressure release device 34 is activated substantially instantaneously releasing treatment fluid 42 and the accumulated pressure through port 36 at zone 26 b, as shown in FIG. 4. This sudden release of treatment fluid 42 under pressure creates fractures 44, or stimulates pre-existing natural fractures, cleats or other naturally occurring or man-made pathways in zone 26 b. After stimulation of zone 26 b, the extreme overbalance stimulation may be conducted on subsequent formation zones 26 without requiring the removal of coiled tubing 16 or assembly 32 from wellbore 18. Additionally, the stimulation of multiple zones does not require the making or breaking of a jointed tubing string. The present invention utilizes coiled tubing 16 as the conduit for the stimulation.

FIGS. 5 and 6 are illustrations of another embodiment of the stimulation system of the present invention, generally denoted by the numeral 10. Illustrated is a horizontal wellbore 18, having an open hole section 46, in a carbonate formation. A portion of wellbore 18 is cased 30 and the well may include jointed tubing 50. The well is designated for stimulation of the targeted formation zone 26 a. Open hole section 46 may have only one pay zone 26 a of interest, or multiple zones. The prior art stimulation systems require large amounts of costly treatment fluids and a may require higher horsepower to stimulate the target zone as well as the formation surrounding the uncompleted portion of wellbore 18 that is not of interest.

Overbalance stimulation system 10 of the present invention provides a cost effective method for selectively stimulating zone 26 a. Bottom hole assembly 32 includes a pressure release device 34 and a perforating gun 48. Assembly 32 is functionally connected and carried by coiled tubing 16. Assembly 32 is positioned adjacent to the target zone 26 a with the pressure release device 34 in the closed position maintaining fluid and pressure within coiled tubing 16.

Pressure is accumulated in coiled tubing 16. The pressure may be accumulated by first injecting a volume of a first treatment fluid 42 a, such as acid, into coiled tubing 16. Desirably first treatment fluid is a liquid. A second treatment fluid 42 b, is pumped into coiled tubing 16 to pressurize the treatment fluid. Desirably, the second treatment fluid is a gas such as, but not limited to, nitrogen. The desired pressure of treatment fluid 42 may be equal to, or greater than, the fracture pressure of formation 26 a.

Once the desired pressure of treatment fluid 42 is achieved, zone 26 a may be stimulated. Perforating gun 32 is detonated. The detonation of gun 32 initiates fractures 44 by creating perforations. Substantially simultaneous with the detonation of gun 32, treatment fluid 42 is substantially instantaneously released under pressure at formation 26 a, illustrated by the arrows in FIG. 6. The extreme overbalance release of treatment fluid 42 extends fractures 44 connected to the wellbore through the perforation tunnels.

From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a method and system for overbalance stimulation with coiled tubing that is novel and unobvious has been disclosed. Although specific embodiments of the invention have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed embodiments without departing from the spirit and scope of the invention as defined by the appended claims which follow. 

1. A method of selectively stimulating a targeted formation zone from a wellbore surrounded by a formation utilizing coiled tubing as the conduit for the stimulation, the method comprising the steps of: positioning an assembly connected to coiled tubing adjacent a target formation zone; sealing between the coiled tubing and the formation to isolate the target formation zone from the rest of the wellbore; accumulating a desired pressure in the coiled tubing with a treatment fluid, the desired pressure at least equal to the fracture pressure of the target formation zone; and releasing substantially instantaneously the accumulated pressure through the assembly at the target formation zone.
 2. The method of claim 1, further including the step of: releasing the sealing isolating the target formation zone; repositioning the assembly adjacent a subsequent target formation zone; sealing between the coiled tubing and the formation to isolate the subsequent target formation zone from the rest of the wellbore; accumulating pressure in the coiled tubing at least equal to the fracture pressure of the subsequent target formation zone; and releasing substantially instantaneously the accumulated pressure through the assembly at the subsequent target formation zone.
 3. The method of claim 1, wherein the sealing step includes: setting a sealing element between the coiled tubing and the formation above the formation zone targeted; and setting a second sealing element between the coiled tubing and the formation below the formation zone targeted.
 4. The method of claim 2, wherein the sealing step includes: setting a sealing element between the coiled tubing and the formation above the formation zone targeted; and setting a second sealing element between the coiled tubing and the formation below the formation zone targeted.
 5. The method of claim 1, wherein the wellbore adjacent the formation zone targeted includes casing having a least one perforation.
 6. The method of claim 5, wherein the sealing step includes: setting a sealing element between the coiled tubing and the formation above the formation zone targeted; and setting a second sealing element between the coiled tubing and the formation below the formation zone targeted.
 7. The method of claim 6, wherein the sealing elements comprise packers.
 8. The method of claim 1, wherein the wellbore adjacent the formation zone targeted includes a slotted liner.
 9. The method of claim 8, wherein the sealing step includes: setting a sealing element between the coiled tubing and the formation above the formation zone targeted; and setting a second sealing element between the coiled tubing and the formation below the formation zone targeted.
 10. The method of claim 9, wherein the sealing elements comprise packers.
 11. The method of claim 1, wherein the wellbore adjacent the formation zone is not cased.
 12. The method of claim 11, wherein the sealing step includes: setting a sealing element between the coiled tubing and the formation above the formation zone targeted; and setting a second sealing element between the coiled tubing and the formation below the formation zone targeted.
 13. The method of claim 12, wherein the sealing elements comprise packers.
 14. A method of selectively stimulating a targeted formation zone from a wellbore utilizing coiled tubing as the conduit for the stimulation, the method comprising the steps of: positioning an assembly connected to coiled tubing adjacent a target formation zone, the assembly including a perforating gun; accumulating a desired pressure in the coiled tubing with a treatment fluid, the desired pressure at least equal to the fracture pressure of the target formation zone; detonating the perforating gun; and releasing substantially instantaneously the accumulated pressure through the assembly at the target formation zone.
 15. The method of claim 14, wherein the releasing step occurs substantially simultaneously with the detonation of the perforation gun.
 16. The method of claim 14, wherein the step of accumulating comprises: injecting a first treatment fluid into the coiled tubing; and pressurizing the first treatment fluid by a second treatment fluid in the coiled tubing.
 17. The method of claim 15, wherein the step of accumulating comprises: injecting a first treatment fluid into the coiled tubing; and accumulating pressure in the coiled tubing by injecting a second treatment fluid in the coiled tubing.
 18. The method of claim 14, wherein the wellbore proximate the target formation zone is not cased.
 19. The method of claim 14, wherein the wellbore adjacent the formation zone targeted includes a slotted liner.
 20. The method of claim 14, wherein the wellbore adjacent the formation zone targeted includes casing having a least one perforation.
 21. The method of claim 18, wherein: the step of accumulating comprises injecting a first treatment fluid into the coiled tubing, and pressurizing the first treatment fluid by a second treatment fluid in the coiled tubing; and the releasing step occurs substantially simultaneously with the detonation of the perforation gun.
 22. The method of claim 19, wherein: the step of accumulating comprises injecting a first treatment fluid into the coiled tubing, and pressurizing the first treatment fluid by a second treatment fluid in the coiled tubing; and the releasing step occurs substantially simultaneously with the detonation of the perforation gun.
 23. The method of claim 18, wherein: the step of accumulating comprises injecting a first treatment fluid into the coiled tubing, and pressurizing the first treatment fluid by a second treatment fluid in the coiled tubing; and the releasing step occurs substantially simultaneously with the detonation of the perforation gun.
 24. A method of selectively stimulating a targeted formation zone from an uncased portion of a wellbore utilizing coiled tubing as the conduit for the stimulation, the method comprising the steps of: positioning an assembly connected to coiled tubing adjacent a target formation zone; accumulating a desired pressure in the coiled tubing with a treatment fluid including an acid, the desired pressure at least equal to the fracture pressure of the target formation zone; and releasing substantially instantaneously the accumulated pressure through the assembly at the target formation zone. 